Recently, many proposals of applying photonic crystals to semiconductor lasers have been reported. Japanese Patent Laid-Open No. 2000-332351 discloses a light source employing a surface emitting laser in which an active layer including a light-emitting material is provided and a two-dimensional photonic crystal is formed in the vicinity of the active layer. The disclosed photonic-crystal surface emitting laser is one type of Distribution Feedback (DFB) laser and has a resonance mode in the in-plane direction of a substrate. In the two-dimensional photonic crystal, columnar holes are periodically formed in a semiconductor layer, and a distribution of its refractive index has two-dimensional periodicity. With the two-dimensional periodicity, part of light generated in the active layer, which has a particular wavelength, resonates to form standing waves, thus causing laser oscillation. Further, the light is taken out in a direction perpendicular to the laser surface with first-order diffraction, and a thus-obtained laser device operates as the surface emitting laser.
Trial products of the photonic-crystal surface emitting laser have been so far fabricated by using various compound semiconductors. Photonic-crystal surface emitting lasers using nitride semiconductors have also been studied.
When trying to reduce the cost of a nitride semiconductor laser, it is advantageous to employ an inexpensive heterogeneous substrate, e.g., a sapphire substrate, instead of an expensive GaN substrate. However, if crystal growth is developed on a heterogeneous substrate as it is, a difficulty arises in obtaining a high-quality crystal less subjected to transitions. In view of such a difficulty, the so-called ELO (Epitaxial Lateral Overgrowth) technique is often utilized for an improvement of crystal quality. With the ELO technique, some concave-convex structure or some mask structure for selective growth is formed on a substrate or in a compound semiconductor layer to develop crystal growth in the lateral direction, thereby obtaining a crystal less subjected to transitions.
Japanese Patent Laid-Open No. 2000-021789 discloses a structure in which the ELO technique is applied to an edge emitting semiconductor laser.
When the ELO technique is applied to the photonic-crystal surface emitting laser, the following problem occurs which is not caused with the edge emitting laser disclosed in the above-cited Patent Literature 2.
FIG. 9 is a schematic view to explain the structure of the edge emitting semiconductor laser disclosed in the above-cited Japanese Patent Laid-Open No. 2000-021789. In FIG. 9, reference numeral 910 denotes a substrate, reference numeral 930 denotes a mask structure for implementation of the ELO technique, and reference numeral 960 denotes an active layer. In the edge emitting semiconductor laser illustrated in FIG. 9, a resonance direction 992 of laser light and emergent light 990 are both restricted in the in-plane direction parallel to the substrate 910, and no light is emitted in a direction toward the substrate. Therefore, optical characteristics of the mask structure 930 used in the ELO technique do not affect characteristics of a laser device.
On the other hand, FIG. 10 is a schematic view to explain a structure in which the ELO technique is applied to the photonic-crystal surface emitting laser. In the photonic-crystal surface emitting laser, a resonance direction 1092 within a photonic crystal 1070 is the in-plane direction of a substrate 1010, while emergent lights 1090 and 1091 are directed perpendicular to the surface of the substrate 1010. The emergent light 1091 directing toward the substrate side reaches the mask structure 1030 that is provided for implementation of the ELO technique, and reflection, diffraction, etc. are generated by a concave-convex structure that is constituted by both the mask structure 1030 and the substrate 1010. Hence, optical characteristics of the mask structure 1030 greatly affect characteristics of a laser device. More specifically, depending on the refractive index of a material forming the mask structure 1030, reflectivity is increased in comparison with the case where the mask structure is not provided. Because reflected light becomes return light or stray light, degradation or instability of the laser characteristics may be caused with the reflected light.